1. Technical Field of the Invention
The present invention relates to a transmission power control system and a transmission power control method in code division multiple access system.
2. Description of the Related Art
In the technical field of mobile communication, the code division multiple access (CDMA) system attracts attention as a technique for effectively utilizing a frequency band. A base station of the CDMA system needs a very large processing gain for receiving a desired radio signal transmitted from a mobile station locating at a far from the base station, and for eliminating an undesired radio signal, which interferes with receiving the desired radio signal, transmitted from a mobile station locating at a near from the base station.
For solving this problem on distance, there is a transmission power control (TPC) method. In this method, transmission power of each mobile station is controlled so that a base station receives the same level of power from any mobile station which is locating anywhere in a service area.
As a technique in relation to this transmission power control in mobile communication, Japanese Patent Application Laid-open No. 125604/1996 discloses a transmission power control method in which it is judged whether a signal to interference ratio (SIR: the ratio of the received power of a desired signal to the received power of an interference signal) calculated in a base station is larger than a predetermined SIR or not, transmission power control bits as a result of the judgement is spread in spectrum into a signal of a transmission power control channel, and the signal of the transmission power control channel is transmitted in parallel with a signal of a communication channel. In addition, Japanese Patent Application Laid-open No. 327073/1997 discloses a method of arranging and transmitting pilot channels for effectively allocating a pilot channel to forward radio channels, which are time division multiplexed, and further for reducing the power of a pilot channel interfering to another cell.
In parallel with the above disclosed techniques, a technique of a transmission power control method is disclosed in "1997 General Meeting Convention Record of the Institute of Electronics, Information and Communication Engineers, Communication 1, B-5-81" (hereinafter called prior art).
A high-speed closed loop control system disclosed in the prior art is shown in FIG. 12. A transmission power control method according to this system concerns a forward link but a similar control method is also applied to a reverse link.
FIG. 12 shows a state that a mobile station 901 establishes communication channels by six connections (multi-connection) at the same time between the mobile station 901 and a base station 902 to perform a multi-code transmission. Such a multi-code transmission is applied in case of performing a simultaneous transmission of a voice and data, a high-speed data transmission with parallel connections, or the like. An individual spread code is assigned to each of the connections.
This system comprises the mobile station 901, base station 902 and base station control apparatus 903. Communication channels corresponding to the multi-connection between the mobile station 901 and base station 902 are established between the base station 902 and a base station control apparatus 903. The base station 902 comprises an amplification part 904 and modem cards MCs (MC1 to MC6 are shown in FIG. 12) corresponding to the respective connections (the number of connections is six in FIG. 12). The base station control apparatus 903 comprises diversity hand-over trunks (DHT1 to DHT6 are shown in FIG. 12) corresponding to the connections, a quality measurement part 907 and an aimed-SIR determination part 908.
The modem cards (MC1 to MC6) of the base station 902 have functions of performing transmission and reception of radio signals between the base station 902 and the mobile station 901 corresponding to the respective connections, and each comprises a reception part and a transmission part of the signals according to CDMA system of the prior art.
In this reception part of each modem card, the signal to interference ratio (SIR) is measured in received signals (measured SIR), and it is compared with an aimed SIR which has been predetermined separately. With the result of the comparison, a TPC (transmission power control) bit pattern for controlling the transmission power is determined which pattern is to be transmitted to the mobile station 901 in addition to a forward link signal transmitted from the corresponding transmission part. For example, when the measured SIR is smaller than the aimed SIR, a request for increasing the reverse transmission power is given to the mobile station 901 with the TPC bits. Conversely, when the measured SIR is larger than the aimed SIR, a request for decreasing the reverse transmission power is given to the mobile station 901 with the TPC bits.
The minimum value of the SIR, which is necessary to meet a predetermined communication quality, is used as the aimed SIR in the high-speed closed loop control. The communication quality is determined on the basis of the frame error rate (FER) in contents of data of received reverse signals. For some reasons such as a fluctuation of propagation characteristics of radio signals, there is a case that the predetermined communication quality is not attained even when the measured SIR obtained from a measurement level of received signals meets the aimed SIR. In contrast with this, when the measured SIR does not reach the aimed SIR, there is a case that the predetermined communication quality is obtained.
For this reason, in consideration of a situation that the mobile station 901 is also in communication with another base station (not shown) by soft hand-over or the like, the base station control apparatus 903 carries out site diversity synthesis of received signals with the other base station corresponding to respective connections by the diversity hand-over trunks (DHT1 to DHT6). Further, the base station control apparatus 903 takes in the received signals after the diversity synthesis to the quality measurement part 907, where the communication quality is measured. The aimed SIR determination part 908 controls to predetermine the aimed SIR at regular intervals on the basis of the measured communication quality. For example, the aimed SIR is increased when the communication quality deteriorates to be less than a predetermined value, and the aimed SIR is decreased when the communication quality is superior to the predetermined value, so that it is controlled to obtain the predetermined communication quality.
Besides, in the base station control apparatus 903, the control of the aimed SIR is performed in consideration of all connections corresponding to the mobile station 901 because the relation between the channels corresponding to the connections established when the respective connections of the multi-connection are established and the mobile station 901 using them is under control of a central processing unit (not shown). A common aimed SIR regulated for all of those connections is sent to the modem cards (MC1 to MC6) of the base station 902.
A conventional transmission power control in a multi-connection is based on an instruction from the base station control apparatus 903 on the assumption that the communication qualities in all connections constituting the multi-connection are the same. That is, as shown in FIG. 13, pilot bits to add to a forward transmission signal and TPC bits (pilot+TPC bits are shown in FIG. 13) are determined on the basis of a measurement result of SIR by the modem card MC1, and added to the transmission signal of the corresponding first connection. The pilot bits are used in a frame synchronism establishment process.
In this manner, by making the pilot+TPC bits common to all connections, it becomes possible to set a large transmission power of the pilot+TPC bits per connection as shown in FIG. 14. As a result, in control of each connection, accuracy in transmission power control or accuracy in channel estimation with the pilot bits can be improved. Besides, since the transmission power of the common pilot+TPC bits can be set to be smaller than the other bits of the signal in which the connections are synthesized, the interference power to other mobile station can be reduced.
In the conventional control of the aimed SIR in this multi-connection transmission, however, there are the following problems.
Because a common value as the aimed SIR set for the modem cards is used for all connections, there is a difference in the communication qualities of received signals of the connections measured in the quality measurement part 907, the aimed SIR determination part 908 becomes hard to determine an aimed SIR in order that all connections are of a predetermined communication quality. For example, when the aimed SIR is increased on the basis of the connection of the worst communication quality, the other connections have excessive reverse transmission powers so the interference power to other mobile station increases. In contrast with this, when the aimed SIR is decreased on the basis of the connection of the best communication quality, the other connections are of less than the predetermined communication quality. Further, a similar problem arises even when the aimed SIR is controlled on the basis of the mean communication quality of all connections.